U.S. patent application number 10/483713 was filed with the patent office on 2004-12-02 for method and apparatus for measuring speed.
Invention is credited to Luscombe, Andrew.
Application Number | 20040239528 10/483713 |
Document ID | / |
Family ID | 3830282 |
Filed Date | 2004-12-02 |
United States Patent
Application |
20040239528 |
Kind Code |
A1 |
Luscombe, Andrew |
December 2, 2004 |
Method and apparatus for measuring speed
Abstract
The invention provides a method and apparatus for monitoring a
moving object, the method comprising: emitting electromagnetic
radiation so that the object passes through the electromagnetic
radiation; detecting a plurality of reflection events, each
comprising a reflection of the electromagnetic radiation from the
object; determining a direction of each of the reflection events
from the direction of emission of the electromagnetic radiation or
from the direction of detection of the respective reflection event;
and determining the relative timing of the reflection events;
wherein information pertaining to the object can be determined, the
information comprising any one or more of a speed, a location and a
direction of the object.
Inventors: |
Luscombe, Andrew;
(Greensborough Vic, AU) |
Correspondence
Address: |
STITES & HARBISON PLLC
1199 NORTH FAIRFAX STREET
SUITE 900
ALEXANDRIA
VA
22314
US
|
Family ID: |
3830282 |
Appl. No.: |
10/483713 |
Filed: |
June 2, 2004 |
PCT Filed: |
July 12, 2002 |
PCT NO: |
PCT/AU02/00944 |
Current U.S.
Class: |
340/942 ;
250/200; 340/556 |
Current CPC
Class: |
G01S 17/50 20130101;
G01P 3/68 20130101; G08G 1/052 20130101; G01S 17/06 20130101 |
Class at
Publication: |
340/942 ;
340/556; 250/200 |
International
Class: |
G08G 001/04 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 12, 2001 |
AU |
PR 6318 |
Claims
1. A method of monitoring a moving object having a velocity,
comprising: emitting electromagnetic radiation so that said object
passes through said electromagnetic radiation; detecting a
plurality of reflection events, each comprising a reflection of
said electromagnetic radiation from said object; determining a
direction of each of said reflection events from the direction of
emission of said electromagnetic radiation or from the direction of
detection of said respective reflection event; and determining the
relative timing of said reflection events; wherein information
pertaining to said object can be determined, said information
comprising any one or more of a speed, a location and a direction
of said object.
2. A method as claimed in claim 1, including deriving further
information from said information, such as that the object has
indeed passed into or through said electromagnetic radiation.
3. A method as claimed in claim 1, wherein said electromagnetic
radiation comprises a plurality of beams of electromagnetic
radiation each having a known emission direction, wherein the
direction of a reflection event is determined from said known
emission directions.
4. A method as claimed in claim 3, wherein said method includes
emitting three or more beams of electromagnetic radiation, and said
velocity has a respective component parallel to each of said
beams.
5. A method as claimed in claim 3, wherein at least one pair of
said beams are non-parallel with respect to each other.
6. A method as claimed in claim 3, wherein each of said beams is a
spatially dispersing beam in a first plane and relatively narrow in
a second plane perpendicular to said first plane.
7. A method as claimed in claim 3, wherein each of said beams is a
composite beam comprising multiple sub-beams.
8. A method as claimed in claim 1, including detecting said
reflection events by means of a plurality of detectors each having
a known detection direction, wherein the direction of a reflection
event is determined from said known detection directions.
9. A method as claimed in claim 8, including detecting said
reflection events by means of three or more detectors each having a
known detection direction.
10. A method as claimed in claim 9, wherein each of said plurality
of detectors has a detection region that is relatively broad in a
first plane and relatively narrow in a second plane perpendicular
to said first plane.
11. A method as claimed in claim 10, wherein said first plane is
substantially upright.
12. A method as claimed in claim 1, wherein said electromagnetic
radiation is infrared radiation or visible light or ultraviolet
light.
13. A method as claimed in claim 1, including modulating said
electromagnetic radiation so that reflected electromagnetic
radiation detected from said reflection events is correspondingly
modulated, and discriminating between said modulated reflected
electromagnetic radiation and background or other undesired
electromagnetic radiation.
14. A method as claimed in claim 1, including DC filtering
reflected electromagnetic radiation detected from said reflection
events or a signal corresponding thereto, to eliminate unmodulated
and therefore undesired detected electromagnetic radiation.
15. A method as claimed in claim 1, including detecting one or more
of said reflection events in the form of at least one temporal
spectrum of reflected electromagnetic radiation intensity.
16. A method as claimed in claim 1, including emitting said
electromagnetic radiation and detecting said plurality of
reflection events from respective locations of close proximity.
17. A method as claimed in claim 1, wherein said electromagnetic
radiation is emitted from one or more sources of electromagnetic
radiation, and said method includes detecting said reflection
events by-means of one or more detectors each having a known
detection direction, wherein each of said one or more sources is
located in close proximity to at least one of said detectors.
18. A method as claimed in claim 17, including detecting reflection
events due to reflection of electromagnetic radiation from a
respective one of said one or more sources by means of a respective
one of said detectors, wherein said respective one of said one or
more sources and said respective one of said detectors are in close
proximity.
19. A method as claimed in claim 17, including detecting said
reflection events, by means of three or more detectors.
20. A method for measuring the speed of a moving motor vehicle,
including the method of claim 1.
21. An apparatus for monitoring an object moving with a velocity
along a path, comprising: an electromagnetic radiation emitter for
emitting electromagnetic radiation into said path; an
electromagnetic radiation detector for detecting a plurality of
reflection events, each comprising a reflection of said
electromagnetic radiation from said object, and outputting an
output signal in response thereto; and computation means for
receiving said output signal, determining the relative timing of
said reflection events and associating with each of said reflection
events a direction derived from the direction of emission of said
electromagnetic radiation or from the respective direction of
detection of said respective reflection event.
22. An apparatus as claimed in claim 21, wherein said computation
means is operable to determine information from at least the
relative timing of said reflection events and said directions
thereof, said information comprising any one or more of a speed, a
location and a direction of said object.
23. An apparatus as claimed in claim 21, wherein said computation
means is operable to derive further information from said
information, such as that the object as indeed passed into or
through said electromagnetic radiation.
24. An apparatus as claimed in claim 21, wherein said emitter
includes a plurality of electromagnetic radiation sources, each for
emitting electromagnetic radiation having a known emission
direction, wherein the direction of a reflection event is
determinable from said known emission directions.
25. An apparatus as claimed in claim 24, wherein said apparatus
includes three or more electromagnetic radiation sources, and said
velocity has a respective component parallel to each of said
emission directions.
26. An apparatus as claimed in claim 24, wherein at least one pair
of said emission directions are non-parallel with respect to each
other.
27. An apparatus as claimed in claim 24, wherein each of said
sources emits a spatially dispersing beam in a first plane and
relatively narrow in a second plane perpendicular to said first
plane.
28. An apparatus as claimed in claim 27, wherein said first plane
is substantially upright.
29. An apparatus as claimed in claim 21, wherein said
electromagnetic radiation detector comprises a plurality of
photodetectors each having a known detection direction, wherein the
direction of a reflection event is determinable from said known
detection directions.
30. An apparatus as claimed in claim 29, wherein said
electromagnetic radiation detector comprises three of more
photodetectors each having a known detection direction.
31. An apparatus as claimed in clam 29, wherein each of said
plurality of photodetectors has a detection region that is
relatively broad in a first plane and relatively narrow in a second
plane perpendicular to said first plane.
32. An apparatus as claimed in claim 21, including at least one
modulator for modulating said electromagnetic radiation so that
said reflected electromagnetic radiation is correspondingly
modulated, and a discriminator for discriminating between said
modulated reflected electromagnetic radiation and background or
other undesired electromagnetic radiation.
33. An apparatus as claimed in claim 21, including at least one DC
filter for filtering said reflected electromagnetic radiation or a
signal corresponding thereto, to eliminate unmodulated and
therefore undesired detected electromagnetic radiation.
34. An apparatus as claimed in claim 21, wherein said apparatus is
operable to detect one or more of said reflection events in the
form of at least one temporal spectrum of reflected electromagnetic
radiation intensity.
35. An apparatus as claimed in claim 21, wherein said emitter and
said detector are located so that said reflection events are
detected at respective locations in close proximity to where said
electromagnetic radiation is emitted.
36. An apparatus as claimed in claim 21, wherein said emitter
comprises one or more sources of electromagnetic radiation, and
said detector comprises one or more photodetectors each having a
known detection direction, wherein each of said one or more sources
is located in close proximity to at least one of said
photodetectors.
37. An apparatus as claimed in claim 36, wherein said apparatus is
configured to detect reflection events due to reflection of
electromagnetic radiation from a respective one of said one or more
sources by means of a respective one of said photodetectors,
wherein said respective one of said one or more sources and said
respective one of said photodetectors are in close proximity.
38. An apparatus as claimed in claim 35, wherein said detector
comprises three or more photodetectors.
39. An apparatus for measuring the speed of a moving motor vehicle,
including the apparatus as claimed in claim 21.
40. A method as claimed in claim 6, wherein said first plane is
substantially upright.
41. A method as claimed in claim 1, including tracking said object
by determining a plurality of values, each at a different time, of
the speed of said object or of the location of said object or of
both the speed and location of said object.
42. A method as claimed in claim 6, wherein said beams do not all
intersect along a single line.
43. A method as claimed in claim 6, including employing further
beams in addition to the minimum number of beams necessary to
determine said information, whereby additional information provided
by said further beams is used for error checking or to improve
reliability or to track said object.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a method and apparatus for
measuring speed, of particular but by no means exclusive
application in monitoring the motion of motor vehicles. In a
preferred embodiment, the invention provides a roadside sensor
system for measuring the speed of motor vehicles.
BACKGROUND OF THE INVENTION
[0002] One particular example of the use of speed measurement
apparatuses is in traffic management and law enforcement, which
often require that traffic be monitored on public roads for
breaches of speed limits. In addition, organisations responsible
for various aspects of traffic flow may desire to gather accurate
information about that flow for many purposes, including planning
road construction, scheduling traffic signals, managing the
deployment of law-enforcement resources, and enforcing the law.
[0003] A variety of existing sensors have been in use for a number
of years for measuring the speed of vehicles from remote locations,
as those vehicles pass a point along the road. Many of these
existing sensors require the installation of pressure-sensing
strips and/or inductive wire loops in the road surface. Others send
a beam of light or electromagnetic radiation towards a motor
vehicle, and determine the speed based on the returned signals.
Various types of devices are suitable for various applications. For
example, in law-enforcement applications measurement systems with a
high degree of confidence in the measured speed are required.
Sometimes portability is a desired characteristic, while at other
times a long and continuous functional lifetime is of prime
concern.
[0004] Thus, although system requirements depend on application,
existing sensor systems used in conjunction with cameras to take
pictures of speeding vehicles generally aim to provide, inter
alia:
[0005] a) highly reliable results,
[0006] b) appropriate positioning of vehicle within the picture
(i.e. the number plate well within the camera's field of view),
[0007] c) continuous, extended unmanned operation,
[0008] d) operation on all road surfaces and types, and
[0009] e) monitoring of speed of the full range of vehicles
including motorbikes and trucks.
[0010] Many systems in current use do not provide all of these
characteristics. Some systems cannot be installed on certain types
of road surfaces, others trigger off the front of vehicles or off
the front wheels and so the position of the rear of long vehicles
is out of camera shot even though the camera may view the vehicles
from the rear, while others are not always triggered by very light
vehicles, such as motor bikes.
SUMMARY OF THE INVENTION
[0011] The present invention provides, therefore, a method of
monitoring a moving object having a velocity, comprising:
[0012] emitting electromagnetic radiation so that said object
passes through said electromagnetic radiation;
[0013] detecting a plurality of reflection events, each comprising
a reflection of said electromagnetic radiation from said
object;
[0014] determining a direction of each of said reflection events
from the direction of emission of said electromagnetic radiation or
from the direction of detection of said respective reflection
event; and
[0015] determining the relative timing of said reflection
events;
[0016] wherein information pertaining to said object can be
determined, said information comprising any one or more of a speed,
a location and a direction of said object.
[0017] Thus, the direction of a reflection event can be determined
from the direction in which the radiation is emitted, or from the
direction from which reflected radiation is detected. In principle,
both could be used to reduce the detection of false events.
[0018] The method can also include deriving further information
from said information, such as that the object has indeed passed
into or through said electromagnetic radiation.
[0019] Preferably said electromagnetic radiation comprises a
plurality of beams of electromagnetic radiation each having a known
emission direction, wherein the direction of a reflection event is
determined from said known emission directions. More preferably
said method includes emitting three or more beams of
electromagnetic radiation, and said velocity has a respective
component parallel to each of said beams. Preferably at least one
pair of said beams are non-parallel with respect to each other.
[0020] Preferably each of said beams is a spatially dispersing beam
in a first plane and relatively narrow in a second plane
perpendicular to said first plane. Preferably said first plane is
substantially upright.
[0021] Each of said beams may be a composite beam comprising
multiple sub-beams.
[0022] Alternatively, said method includes detecting said
reflection events by means of a plurality of detectors each having
a known detection direction, wherein the direction of a reflection
event is determined from said known detection directions. More
preferably said method includes detecting said reflection events by
means of three or more detectors each having a known detection
direction.
[0023] Preferably said each of said plurality of detectors has a
detection region that is relatively broad in a first plane and
relatively narrow in a second plane perpendicular to said first
plane. Preferably said first plane is substantially upright.
[0024] Preferably said electromagnetic radiation is infra-red
radiation. In other embodiments, however, said electromagnetic
radiation is visible light or ultraviolet light.
[0025] Preferably said electromagnetic radiation is modulated so
that reflected electromagnetic radiation detected from said
reflection events is correspondingly modulated, and said method
includes discriminating between said modulated reflected
electromagnetic radiation and background or other undesired
electromagnetic radiation.
[0026] Preferably said method includes DC filtering reflected
electromagnetic radiation detected from said reflection events or a
signal corresponding thereto, to eliminate unmodulated and
therefore undesired detected electromagnetic radiation.
[0027] Preferably said method includes detecting one or more of
said reflection events in the form of at least one temporal
spectrum of reflected electromagnetic radiation intensity.
[0028] Thus, a particular prominent reflection event (perhaps
corresponding to a reflective number plate) could be selected for
use, providing a clearer indication of the passage of a particular
object.
[0029] Preferably said method includes emitting said
electromagnetic radiation and detecting said plurality of
reflection events from respective locations of close proximity.
[0030] In one embodiment, said electromagnetic radiation is emitted
from one or more sources of electromagnetic radiation, and said
method includes detecting said reflection events by means of one or
more detectors each having a known detection direction, wherein
each of said one or more sources is located in close proximity to
at least one of said detectors. More preferably said method
includes detecting reflection events due to reflection of
electromagnetic radiation from a respective one of said one or more
sources by means of a respective one of said detectors, wherein
said respective one of said one or more sources and said respective
one of said detectors are in close proximity.
[0031] Preferably said method includes detecting said reflection
events by means of three or more detectors.
[0032] It will be understood that a particular detector could be
used as the detector for more than one source. The close
positioning of the sources and detectors assists in eliminating the
adverse effects of shadows and glare points. Modulating the
electromagnetic radiation reduces the effects of shadows and glare
points from ambient light sources, while this close positioning
reduces the possibility of shadows from the sources and the
negative effects of glare points (as, if viewing a scene from the
only source of illumination, directly cast shadows should not be
visible).
[0033] In one aspect, the invention provides a method for measuring
the speed of a moving motor vehicle, according to the method
described above.
[0034] The present invention also provides an apparatus for
monitoring an object moving with a velocity along a path,
comprising:
[0035] an electromagnetic radiation emitter for emitting
electromagnetic radiation into said path;
[0036] an electromagnetic radiation detector for detecting a
plurality of reflection events, each comprising a reflection of
said electromagnetic radiation from said object, and outputting an
output signal in response thereto; and
[0037] computation means for receiving said output signal,
determining the relative timing of said reflection events and
associating with each of said reflection events a direction derived
from the direction of emission of said electromagnetic radiation or
from the respective direction of detection of said respective
reflection event.
[0038] Preferably said computation means (typically a suitably
programmed computer or hardware device) is operable to determine
information from at least the relative timing of said reflection
events and said directions thereof, said information comprising any
one or more of a speed, a location and a direction of said
object.
[0039] The detector may include signal processing means for
processing said output signal before outputting said output
signal.
[0040] The computation means may also be operable to derive further
information from said information, such as that the object has
indeed passed into or through said electromagnetic radiation.
[0041] Preferably said emitter includes a plurality of
electromagnetic radiation sources, each for emitting
electromagnetic radiation having a known emission direction,
wherein the direction of a reflection event is determinable from
said known emission directions. More preferably said apparatus
includes three or more electromagnetic radiation sources, and said
velocity has a respective component parallel to each of said
emission directions. Preferably at least one pair of said emission
directions are non-parallel with respect to each other.
[0042] Preferably each of said sources emits a spatially dispersing
beam in a first plane and relatively narrow in a second plane
perpendicular to said first plane. Preferably said first plane is
substantially upright.
[0043] Each of said beams may be a composite beam comprising
multiple sub-beams.
[0044] Alternatively, said electromagnetic radiation detector
comprises a plurality of photodetectors each having a known
detection direction, wherein the direction of a reflection event is
determinable from said known detection directions. More preferably
said electromagnetic radiation detector comprises three of more
photodetectors each having a known detection direction.
[0045] Preferably said each of said plurality of photodetectors has
a detection region that is relatively broad in a first plane and
relatively narrow in a second plane perpendicular to said first
plane. Preferably said first plane is substantially upright.
[0046] Preferably said electromagnetic radiation is infra-red
radiation. In other embodiments, however, said electromagnetic
radiation is visible light or ultraviolet light.
[0047] Preferably said apparatus includes at least one modulator
for modulating said electromagnetic radiation so that said
reflected electromagnetic radiation is correspondingly modulated,
and a discriminator for discriminating between said modulated
reflected electromagnetic radiation and background or other
undesired electromagnetic radiation.
[0048] Preferably said apparatus includes at least one DC filter
for filtering said reflected electromagnetic radiation or a signal
corresponding thereto, to eliminate unmodulated and therefore
undesired detected electromagnetic radiation.
[0049] Preferably said apparatus is operable to detect one or more
of said reflection events in the form of at least one temporal
spectrum of reflected electromagnetic radiation intensity.
[0050] Preferably said emitter and said detector are located so
that said reflection events are detected at respective locations in
close proximity to where said electromagnetic radiation is
emitted.
[0051] In one embodiment, said emitter comprises one or more
sources of electromagnetic radiation, and said detector comprises
one or more photodetectors each having a known detection direction,
wherein each of said one or more sources is located in close
proximity to at least one of said photodetectors. More preferably
said apparatus is configured to detect reflection events due to
reflection of electromagnetic radiation from a respective one of
said one or more sources by means of a respective one of said
photodetectors, wherein said respective one of said one or more
sources and said respective one of said photodetectors are in close
proximity.
[0052] Preferably said detector comprises three or more
photodetectors.
[0053] In one aspect, the invention provides an apparatus for
measuring the speed of a moving motor vehicle, including the
apparatus described above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0054] In order that the present invention may be more clearly
ascertained, preferred embodiments will now be described, by way of
example, with reference to the accompanying drawing, in which:
[0055] FIG. 1 is a schematic view of a sensor system according to a
preferred embodiment of the present invention with a typical field
of view thereof, viewed from above;
[0056] FIG. 2 shows an example measurement plane angling across a
road of the system of FIG. 1;
[0057] FIG. 3 shows an example minimal arrangement of measurement
planes for the system of FIG. 1, viewed from above;
[0058] FIG. 4 is a signal-flow diagram showing the elements used to
monitor one measurement plane by means of the system of FIG. 1, and
the signals that flow therebetween;
[0059] FIG. 5 is a schematic view of the housing with cylindrical
lenses, photodiodes and emitting diodes of a high precision sensor
system according to a further embodiment of the present
invention;
[0060] FIG. 6 is a circuit diagram of a representative
trans-impedance amplifier circuit of the type to which each of the
photodiodes of the system of FIG. 5 is connected;
[0061] FIG. 7 is a half circuit diagram of a representative circuit
of the type to which each light emitting element of the system of
FIG. 5 is connected;
[0062] FIG. 8 is a plot of a time record of a car passing through a
measurement plane, measured with the system of FIG. 5; and
[0063] FIG. 9 is a schematic diagram illustrating how speed can be
calculated with the system of FIG. 5.
DETAILED DESCRIPTION
[0064] A sensor system for measuring the speed of motor vehicles
according a preferred embodiment of the present invention is shown
generally at 10 in situ beside a road 12 in FIG. 1 from above.
[0065] Broadly, the sensor system 10 includes a number of
light-emitting elements for directing beams of light 14 towards
vehicles on the road 12, light collecting elements for collecting
light reflected from those vehicles and light-sensing elements for
detecting the light collected by the lenses. The light collecting
elements (as will be discussed in more detail below) are selected
to collect or focus light substantially from the direction of one
plane only. In this embodiment each comprises a cylindrical lens
optically behind an aperture in the form of a vertical slit, though
a cylindrical lens alone may also be suitable in some applications.
The lenses, slits and light-sensing elements are arranged so that
each light-sensing element receives the light from only one of the
lenses, so that each light-sensing element collects light from a
limited spatial region. The lenses are oriented to collect light
from a detection region comprising a narrow, vertical plane,
generally forward of the sensor 10. The light-sensing elements are
also positioned appropriately relative to the lenses to accentuate
the lateral narrowness of these detection regions. It will be
understood that, although these detection regions are referred to
as planar they will not be precisely so, and--indeed--each
detection region requires suitable width to receive sufficient
light for detection to occur. It is a straightforward matter,
however, to adjust this width according to requirements, such as
the sensitivity of the light-sensing elements, typical ambient
lighting, distance of the sensor system 10 from the vehicles, etc.
For convenience, however, the detection regions are referred to
below as measurement planes; such a representative plane is shown
schematically at 16 in FIG. 2, which is a perspective view
equivalent to FIG. 1. Also shown in FIG. 2 is a pole 18 on which
the sensor system 10 is mounted.
[0066] The system 10 is used with a computational device (typically
a programmed computer: not shown) for receiving from the sensor
system 10 signals indicative of the light sensed by the system 10,
and performing the required analysis to determine vehicle speed.
The analysis phase is also discussed in further detail below.
[0067] The light-emitting elements are located near the lenses
shining towards the sensor system's field of view so that each
light-sensing element is especially sensitive to retro-reflective
objects in its respective measurement plane. Each light-emitting
element has a control input which allows the brightness of emitted
light to be controlled. This control input enables insensitivity to
ambient lighting to be achieved, if it is required. When the
brightness of the emitted light is varied in a known way, the
computational device can distinguish between the components of the
signals due to ambient light and components due to retro-reflected
light from a vehicle (i.e. originating from the light-emitting
elements and collected by the lenses). If only the component due to
the retro-reflected light is used in calculations, then calculation
of vehicle speed by the computational device is relatively simple
and highly reliable. This is because retro-reflecting objects on
the back and front of vehicles (viz. number plates, reflectors and
the like) generally have relatively simple shapes, and the light
patterns reflected by them to the sensor system 10 are also
relatively simple and have a relatively straightforward dependence
on vehicle speed and position.
[0068] The light-emitting elements need not have narrow beam widths
(although that might be advantageous in some applications), as the
narrowness of the measurement planes allows the direction of the
source of reflected light to be determined to the accuracy implied
by the width of each measurement plane.
[0069] Although a majority of vehicles travel straight along the
road and will pass through the measurement planes at a known and
constant angle, some vehicles may not. For example, vehicles might
be changing lanes or swerving slightly for some other reason as
they pass through the measurement planes. The use of multiple
measurement planes addresses this phenomenon, angled to the first.
Each measurement plane is added by adding--as required--further
light-emitting devices, lenses, slits and light-sensing elements.
Thus, when a second (third, etc.) light-sensing element, slit, and
light-emitting element are added to the first one but offset from
it to the left but so that the second measurement plane so produced
is parallel to the first, the variations in light sensed by the two
light-sensing elements is similar, but the second is delayed
relative to the first. Knowing the distance between the measurement
planes and the angle of the planes to the road, it is possible to
calculate the vehicle's speed from the time delay between the two
outputs of the light-sensing elements, that is, by means of a time
over distance calculation.
[0070] FIG. 3, therefore, is a plan view of a typical installation
of the sensor system 10 with multiple measurement planes 20, 22,
24, 26; the locations of the measurement planes 20, 22, 24, 26 are
determined by the physical locations of the lenses and
light-sensing elements. These measurement planes 20, 22, 24, 26
define a field of view of the sensor system 10. The measurement
planes 20, 22, 24, 26 are offset relative to each other by known
distances and angles, and do not all intersect along a single line.
This allows the time delays between the variations of light
produced on each plane by a retro-reflecting object travelling
through the planes to be translated into a speed, direction, and
distance from the sensor system 10. The sensor system may be
constructed with more than the minimum number of measurement planes
mathematically necessary to perform the calculation of speed,
direction, and distance to give a degree of redundancy to allow for
error checking and reliability. The sensor system 10, as has been
made clear, is constructed with all of its measurement planes 20,
22, 24, 26 vertical (i.e. with their normals parallel to a single
plane), so that the sensor system 10 is not sensitive to the
component of travel in that vertical direction; such an
implementation is simpler and cheaper to construct, but it is
envisaged that--if necessary--a more complex arrangement could
used, possibly with additional measurement planes. This might be
used for measuring airborne vehicles or objects.
[0071] The speed of a vehicle is measured as it passes through this
field of view. The field of view angles across the road (as shown
in FIG. 1 to 3) or--alternatively--down (i.e. essentially along)
the road if the sensor system 10 is mounted above the road 12. The
sensor system 10 would be far less effective when pointing directly
down on traffic or directly across traffic because the number plate
and reflectors are not normally visible from these positions.
[0072] The speed, direction and distance of a vehicle passing
through the field of view can be determined by measuring the time
for the vehicle to travel between the measurement planes 20, 22,
24, 26 within the field of view. Referring to FIG. 2, it will
readily be understood that, as a vehicle travels along the road 12
and passes through the representative measurement plane 16, light
falling on that plane's light-sensing element or elements due to
the vehicle being a different colour and brightness from the
roadway will vary. Because of the positioning of the lenses,
masking slits and light-sensing elements, only the light from the
measurement plane will fall on the respective light-sensing element
or elements.
[0073] However, the light variation will be complex owing to
several factors. One of these factors is shadows from nearby moving
vehicles falling on the vehicles being detected. Such shadows
produce moving patterns of light that are in many ways similar to
those of the vehicle being measured, but the patterns will move at
a speed determined by the vehicle casting the shadow rather than
from the vehicle being monitored. Also, there may be light from
nearby bright lights reflected off the vehicle, that is, "glare
points". Although glare points are lights reflected from the
vehicle, they normally move at a different speed from that of the
vehicle. Their motion is a property of several variables including
the position of the light source, which may itself comprise the
headlights of another moving vehicle, the position of the
light-sensing element, and the shape and motion of the vehicle
being monitored. These complex variations of light patterns due to
factors other than the vehicle's speed can create problems in
reliably processing the signals.
[0074] To avoid these potential problems, the light-emitting
elements have control inputs designed to allow distinct temporal
variations in brightness to be emitted by them so that the
components of the signals sensed by the light-sensing elements due
to the light-emitting elements can be identified and distinguished
from the components due to ambient lighting. The reflection of the
light from the light-emitting elements back to the sensor system is
analogous to the transmission stage of many modulation schemes used
in radio and other electronic and optical communication systems.
FIG. 4 illustrates this situation schematically. In FIG. 4, a
light-emitting element 30 is controlled by means of a control input
32. The light-emitting element 30 directs light 34 into the field
of view and reflected from a vehicle in the field of view according
to its reflectivity. This leads to a multiplication 36 (by a factor
less than 1) of the initial light signal, depending on that
reflectivity. The resulting light signal 38, as well as light from
other sources 40, are collected by the masked lens 42, and
transmitted to the light-sensing element 44. The light-sensing
element 44 then outputs an output signal 46.
[0075] Thus, a proportion of the light emitted by the
light-emitting elements 30 is reflected to the light-sensing
elements 44 by objects intersected by--or passing through--the
measurement planes. For each measurement plane 20, 22, 24, 26, this
proportion varies as objects move through a measurement plane, and
this proportional reflection 36 corresponds to the multiplication
stage in many modulation schemes, such as Amplitude Modulation (as
used in AM radio), Pulse Amplitude Modulation, and various Spread
Spectrum modulation schemes in which the carrier wave has a
relatively complex variation in time. The difference between the
various modulation schemes applicable to the present sensor system
10 is the shape of the carrier waveforms. The carrier waveform of a
modulation scheme corresponds to the signals supplied to the
control inputs 32 of the light-emitting elements 30, the modulation
scheme's signal waveform corresponds to the variation in
reflectivity of the measurement plane as objects pass through it,
and the modulation scheme's transmission waveform corresponds to
the signals output 46 by the light-sensing elements 44. Background
noise in the electromagnetic spectrum of the modulation scheme
corresponds to light sources 40 other than light-emitting elements
30 of the sensor system 10.
[0076] Because the light-emitting elements 30 have control inputs
32, the sensor system 10 can support whichever modulation scheme is
most appropriate for the application. For example, in an
application for gathering approximate traffic-flow statistics at
many points along a road, it may be appropriate to reduce costs by
implementing a simple modulation scheme such as Amplitude
Modulation, even though it may be susceptible to a degree of
interference. In a law-enforcement application, it will be
appropriate to implement a highly secure and reliable type of
modulation, such as one of the spread-spectrum types, even though
it may add cost to the system.
[0077] Returning to FIG. 3, an arrangement with four measurement
planes 20, 22, 24, 26 viewed from above, the two measurement planes
22 and 24 are at angles to the first two 20, 26, and it is possible
to calculate the angle and position of the path of a point on the
vehicle that produces a distinct variation in retro-reflected light
as it travels through the field of view. This can be seen by
referring to the two paths drawn in FIG. 3, first path 50 (parallel
to the road) and second path 52 (oblique to the road). If first
path 50 is followed by a vehicle, it can be seen that the
measurement planes are crossed in a particular order (viz. 20, 22,
24, 26) with a particular ratio of distances between the crossing
points. If second path 52 is followed, both the order of the
crossing points (viz. 22, 20, 24, 26) and the ratio of distances
between crossing points is different. It can also be seen that if
first path 50 is maintained parallel to the road but moved across
the road in either direction (i.e. to be either closer to or
further from the sensor system 10), the distance between the
crossing point of planes 20 and 22 ("Distance 20-22") changes
although that between planes 20 and 26 ("Distance 20-26") does not.
In fact (Distance 20-22)/(Distance 20-26) is proportional to the
distance across the road between first path 50 and the crossing
point of planes 20 and 22. A similar situation exists for (Distance
24-26)/(Distance 22-26), which is proportional to the distance
across the road between first path 50 and the crossing point of
planes 24 and 26, which is in fact at the sensor system 10.
[0078] Examples of points on a vehicle that produce a distinct
variation in retro-reflected light are the edges of reflectors and
number plates. As the edge of a reflector or number plate passes
through each measurement plane, there will be a sharp jump in the
amount of light reflected back to the sensor system. A
computational device receiving the signals from the sensor system
can use these sharp jumps to identify such points. A degree of
error checking is provided with four measurement planes because
there will be more than one point on the vehicle producing a
distinct variation in retro-reflected light. Number plates and
reflectors have more than one edge, and there are normally two
reflectors and a number plate on the rear of a vehicle. This
multiplicity of edges presents an opportunity to make several speed
and path measurements for each vehicle, and the several
measurements so produced can be used for cross checking. Even so,
to allow a higher degree of error checking to be performed, the
sensor system can be constructed with more than the minimum number
of measurement planes. These will provide extra information to any
computational device receiving signals from the sensor system, and
so further cross checking can be performed. By the addition of ever
more measurement planes, it is possible to provide for an
increasing degree of error checking and certainty of measurement
results, although it is envisaged that no more than six measurement
planes would ever be needed if a single motor vehicle speed
measurement is all that is required.
[0079] It is also possible to use extra measurement planes to track
the speed of a vehicle, that is, to measure that speed multiple
times over a distance of several or more metres as the vehicle
travels through the field of view. In an implementation for
tracking, the field of view will generally be larger than one for
taking a single speed measurement per vehicle. This can be achieved
by designing the position of the light-sensing elements and lenses
so that the measurement planes are more spread out. One application
of a tracking sensor is in checking if vehicles stop at a stop
sign, or slow down appropriately at a give-way or yield sign. By
tracking vehicle speed for a few metres around the vicinity of the
sign, in many cases it will be possible to determine if the vehicle
was driven dangerously or illegally.
[0080] A detailed description is now provided on a high precision
sensor system according to the present invention, particularly
adapted for measuring vehicle speed in applications (such as
traffic speed enforcement) requiring a high degree of confidence in
each measurement. In such applications, it is not necessary that
every passing vehicle be measured, but rather that there be a high
degree of confidence in any measurement that is made.
[0081] The sensor system of this embodiment has six measurement
planes to produce signals suitable for calculations of vehicle
parameters (speed, vehicle path, and direction) in the two
dimensions parallel to a road surface. Signals pertinent to the
vertical direction are not produced, although other implementations
of the device could produce such measurements. Though four
measurement planes might suffice in this embodiment, six planes are
included in order to produce redundant signals. These redundant
signals enable a degree of cross checking and confirmation in any
calculations carried out on the signals, and this increases
confidence in the calculation results.
[0082] The basic elements of the sensor system of this embodiment
are comparable to those of the above sensor system 10, but are
described for this embodiment in detail as follows.
[0083] The sensor system includes two lenses that focus
substantially in one direction only, each of which has three light
sensing elements for detecting light passing through the respective
lens. Each lens is glass with a cylindrical focussing surface with
a focal length of 150 mm, and rectangular in profile (50
mm.times.25 mm.times.3.5 mm thick), with the long dimension
parallel to the axis of the cylindrical surface. Glass cylindrical
lenses have the advantage of ready availability, but other
arrangements could be used, including slits, cylindrical mirrors or
suitable Fresnel lenses, as well as alternative shapes of glass
lenses and mirrors, provided that light is gathered substantially
more in one direction than the other. Failure to do this will
result in the measurement planes being relatively thick, and less
useful for the timing and locating purposes. In this embodiment,
the planes may be around 600 mm thick at the more remote vehicles.
They could be wider still (with particular wide roads, for example)
and the sensor system device could still produce useful results,
but the system would thereby be less than optimal. Each application
of the system will impose its own constraints on plane
thickness.
[0084] The optical demands placed on the cylindrical lenses are
less than, for example, imaging applications so lenses constructed
for imaging applications will generally be satisfactory.
[0085] Referring to FIG. 5, which is a schematic view of the
interior of the housing of this sensor system, the cylindrical
lenses 60, 62 are held by the housing and fixed in position
relative to each other. The axes of their cylindrical surfaces are
vertical (out of the page in FIG. 5) and parallel to each other.
The lenses 60, 62 are therefore held at the same distance above the
ground, with a separation 64 of 700 mm. The lenses are oriented so
that their main focal axes are substantially parallel to each other
and approximately perpendicular to the line between them. The
alignment between the axes of the cylindrical surfaces of the two
lenses is important: the vertical alignment of the measurement
planes is primarily determined by the alignment of these axes and,
according to this embodiment, to calculate the required parameters
of vehicle travel it is assumed that the distances between the
measurement planes do not vary in the vertical direction. The axes
should not deviate from each other by more than a centimetre over a
distance of two metres along the axes (i.e. 0.25 mm within the
length of the lenses), or this inaccuracy will become the dominant
one in any calculations based on the outputs of the sensor
system.
[0086] The other alignment requirements are less critical.
Miss-alignment of the two axes of the cylindrical surfaces with the
vertical about a horizontal axis in the direction of the
cylindrical lens's direction of focus results in a speed
calculation error proportional to the sine of the angle. This is
true even though the two axes of the cylindrical surfaces may be
exactly parallel with each other. Alignment within 8 degrees of the
vertical will result in an error in any speed calculations of less
than one percent. For the remaining positioning parameters,
accuracy must be sufficient to allow all measurement planes to be
positioned so that typical vehicles will travel through all of them
within a quarter of a second or so. The distance between the lenses
is a trade off between having a compact housing and providing space
between measurement planes for sufficiently long delays between the
signals from each measurement plane. What is sufficient depends on
many factors including the capability of any device processing the
signals to calculate speeds, and on the width of the planes
themselves. Any distance between 0.5 and 1 m is adequate for this
application. With different electronics and other applications, any
distance from millimetres to tens of metres might be appropriate.
Gross deviations in lens positioning from that described above
might result in a system with measurement planes that typical
vehicles cannot travel through (such as pointing the lenses away
from each other, aiming them too high, or placing them at grossly
different heights), which clearly should be avoided.
[0087] The sensor system has, also within the housing, six light
sensing elements 66a,b,c and 68a,b,c. Each of these consist of one
photodiode (such as a SFH213FA) connected to a trans-impedance
amplifier circuit as shown in FIG. 6. The amplifier together with
the photodiode package forms the light-sensing element. The
SFH213FA has a spherical lens as part of its package. This lens
does not interfere with the cylindrical lenses 60, 62 (described
below) and in fact aids the overall performance of the system.
Additional similar lenses may be fixed in front of the photodiodes
to further aid in gathering light, or if photodiodes packaged
without lenses are used. Such lenses are considered to be part of
the light-sensing elements. It should be noted, however, that the
thickness of the measurement planes at the vehicles increases
proportionally with an increase in this lens diameter (all other
things being held constant), and so a lens with not too large a
diameter relative to the other components should be used. The
SFH213FA package is suitable as manufactured without additional
lenses. Each SFH213FA package should be aimed and fixed in position
to receive the bulk of their incident light from the respective
cylindrical lens 60 or 62. They should be positioned one focal
length (of cylindrical lenses 60, 62) behind cylindrical lenses 60,
62, that is, the distance 70, 72 from the lenses 60, 62 should be
equal--to within a few millimetres--to the focal length (150 mm) of
lenses 60, 62. If closer or further from the cylindrical lenses 60,
62, the measurement planes will become progressively out of focus,
and in effect the measurement planes will become wider (as occurs
when increasing the diameter of the photodiode package lens). A
small degree of de-focus is acceptable since the measurement planes
have a finite thickness, but if the measurement plane thickness is
increased too much, there can be difficulties in making time
calculations. For each cylindrical lens 60, 62, the three
photodiodes 66a,b,c and 68a,b,c respectively are positioned in a
horizontal line centred at 5 mm intervals 74 and level with the
middle of the cylindrical lens. This is to give an appropriate
spread of the measurement planes at the traffic. The photodiodes
should be placed to position the measurement planes so that they
overlap the three measurement planes from the other cylindrical
lens somewhere in the-vicinity of the traffic.
[0088] Referring to FIG. 6, the amplifier circuit is designed to
amplify frequencies in the band 50 khz to 1 Mhz and not pass DC
signals. This makes the sensor system insensitive to sunlight and
most other ambient light likely to be encountered. The system can
still be useful operating in other frequency bands, and even with
DC levels passed. It is possible that any devices to which the
sensor system is connected can perform further filtering of the
signals, and in fact other devices could perform any filtering
required. In this embodiment, the band was chosen to avoid large DC
signals, primarily from the sun, from swamping the output signals,
and using up the output voltage range. The final signal to noise
ratio with the particular configuration of components used was
appropriate when designed for this band.
[0089] Also within the housing are two light emitting elements 76
and 78; each straddles a respective cylindrical lens 60, 62. Each
light emitting element 76, 78 comprises 24 HSDL-4230 infra-red
emitting diodes, 12 on each side of the respective cylindrical lens
and arranged in four rows of three (one of which rows of three
diodes is visible in FIG. 5 either side of each cylindrical fens).
These are driven by the circuitry, half of which is shown in FIG. 7
(the other half being identical). The emitting diodes are aimed
parallel to the focusing direction of the cylindrical lenses (to
the right in FIG. 5), that is, at the traffic, and are fixed in
position. It is possible to fix a cylindrical lens in front of
these emitting diodes in a way that focuses the light into the
measurement planes, rather than allow it to spread out and merely
include--and thereby illuminate--the measurement planes. The number
of emitting diodes could then be reduced. This will result in less
total infra-red light being emitted which may make it more
difficult for a driver to detect--and thereby avoid--the sensor
system. The measurement planes can also be further narrowed if the
emitted light is focussed to a width comparable to the measurement
planes or less. It is even possible to use the directional nature
of the transmitted light as the prime means of forming the
measurement planes, rather than relying instead on the known
direction of detected light.
[0090] The light emitting elements 76, 78 draw relatively large
currents when pulsed on, and there is a possibility that a small
ripple voltage may appear on the power supply depending on the
supply's characteristics. This can reduce the accuracy of the light
sensitive elements if they are also powered directly by the same
supply. Adequate filtering or separate supplies should be used if
this is found to be a problem.
[0091] The housing should be situation so that light from the
light-emitting elements 76, 78 reaches the light sensing elements
66a,b,c and 68a,b,c primarily by shining out into the area of the
roadway and reflecting back through the cylindrical lenses 60, 62
onto the light sensing elements 66a,b,c and 68a,b,c. If a small
amount of light travels directly onto the light sensing elements
66a,b,c and 68a,b,c (that is, backwards from the light-emitting
elements 76, 78), that light will add a small constant value to the
signals output by the light sensing elements 66a,b,c and 68a,b,c.
This should not be of great importance, but--depending on the
application--if the direct light is many times that of the
reflected light it may make the signals from the roadway difficult
to detect and the system less useful. In this embodiment, the
light-emitting elements 76, 78 are only 150 mm away from the
light-sensing elements 66a,b,c and 68a,b,c, so--as the traffic may
be only 8 or 12 metres away, it is preferred that measures be taken
to block such direct light before it reaches the light-sensing
elements 66a,b,c and 68a,b,c. Such light, it should be noted,
includes light that reaches the light-sensing elements 66a,b,c and
68a,b,c by diffusing through circuit boards or leaking through
cracks. This problem is readily avoided by providing, within the
housing, suitable baffles or light insulation between the
light-sensing elements 66a,b,c and 68a,b,c and the light-emitting
elements 76, 78.
[0092] To use the sensor system in roadside speed calculations, the
sensor system should be controlled and monitored. Its
light-emitting elements 76, 78 have control inputs, and the signals
from the light-sensing elements 66a,b,c and 68a,b,c need to be
monitored. In this embodiment, the light-emitting elements 66a,b,c
and 68a,b,c are designed to be pulsed on briefly. This enables a
scheme analogous to pulse amplitude modulation to be used. The
controlling electronics can signal either light-emitting element 76
or 78 to pulse at any time, and the amplitude of the resulting
pulse output by the relevant light-sensing elements 66a,b,c,
68a,b,c indicates the proportion of pulsed light retro-reflected by
any objects in the area of the roadway. A micro-controller is
connected to the sensor system, which periodically signals the
light-emitting elements 76, 78 to pulse, and then samples the
pulses output by the light-sensing elements 66a,b,c, 68a,b,c using
an analogue to digital converter. In this way, records over time of
the retro-reflectivity of the objects in each measurement plane as
the objects pass through the measurement plane are stored in the
micro-controller's memory. The micro-controller then analyses them
and determines the times at which highly reflective objects enter
each measurement plane, and from these times calculates vehicle
speed.
[0093] FIG. 8 shows an example time record of a car (a white
station wagon/estate) passing through a measurement plane, plotted
as amplitude A versus time T in seconds. The waveform drawn as a
solid curve 80 is the measured signal (median of 9 samples) coming
from the measurement plane. The waveform drawn with broken curve 82
is the dynamic threshold value calculated according to the peak
timing algorithm presented in Table 1. The three largest measured
peaks 84, 86, 88 in the solid curve 80 are produced respectively by
the two rear reflectors that are part of the tail-light assemblies
on each side of the vehicle, and the number plate. The number plate
on this vehicle was mounted left of centre, so the peak 88 due to
the number plate is not centred between the other two narrower
peaks 84, 86. The bulk of the vehicle is not especially retro
reflective (and certainly not as retro reflective as are the
reflectors and number plates), so it has little effect on the
waveforms. This waveform contains the complete record of the full
length of the vehicle as it travelled through the measurement
plane. Although many suitable algorithms are available for
determining the time at which peaks or pulses occur in signals, and
in this case for monitoring signals from the roadside sensor, that
used in the system of this embodiment is summarized in simplified
form in Table 1. In Table 1, therefore, arrays are assumed to be of
infinite size. In practice, some management of the arrays and array
indexes will be required to prevent the micro-controller's memory
from filling up. Also, the most computationally efficient way of
implementing the algorithm has not been described. As it is
described in Table 1, many operations are repeated that in practice
need not be. This has been done for clarity in describing the
algorithm. The algorithm can be varied to suit different
applications. The minimum threshold value can be varied, the number
of array values used to calculate the averages and medians can be
varied, sample times varied, and so on.
[0094] Speed is calculated as follows. FIG. 9 shows an arrangement
of measurement planes 90 to 100 viewed from above, and a path 102
taken across them by a trigger point on a vehicle.
[0095] If an X axis is defined orthogonal to the Y axis in FIG. 9
(both axes being in the plane of the road), points on the
measurement planes 90 to 100 satisfy the following equations:
y=a.sub.1x for plane 90
y=a.sub.2x for plane 92
y=a.sub.3x for plane 94
y=a.sub.4x+c for plane 96
y=a.sub.5x+c for plane 98
y=a.sub.6x+c for plane 100
[0096] and for the path of the trigger point:
y=Ax+C,
[0097] i.e. the path is assumed to be straight.
[0098] All of the above equations are linear and, provided that the
trigger point on the vehicle travels at constant speed past the
measurement planes, the following equation holds: 1 X i = - T i j T
i m a k j c i m - T i j T i k a jm c ik - a mk c ij T i j T i m a k
j a i m + T i j T i k a jm a ik + a mk a ij
[0099] where: X.sub.i is the x coordinate of the point at which the
trigger path crosses the ith measurement plane; the double
subscript notation refers to a difference between the respective
value (e.g. a.sub.kj=a.sub.j-a.sub.k); T.sub.i T.sub.j T.sub.k and
T.sub.m are the times at which the trigger point crossed each of
four different planes i, j, k, and m; and C.sub.i is either c or 0
depending on which measurement plane the i refers to.
[0100] All six of the x coordinates are calculated by selecting
appropriate sets of four times as measured according to the
algorithm described in Table 1. The a and c values are a function
of the sensor geometry and are stored in the memory of the
micro-controller ready to use. From the X coordinates, the Y
coordinates can be calculated using the measurement plane
equations:
Y.sub.i=a.sub.iX.sub.i+c.sub.i
[0101] and then the distances using the following:
D.sub.ij={square root}{square root over
((Y.sub.j-Y.sub.i).sup.2+(X.sub.j-- X.sub.i).sup.2)}
[0102] and speed v from the following: 2 v = D i j T i j
[0103] If a number of speeds are calculated from different sets of
four measurement planes selected from the six measured times, the
speeds can be checked for consistency. If a set of six time
measurements results in several inconsistent speed values then
there may have been interference or the assumptions about constant
vehicle speed and direction as it crossed the measurement planes
may have been violated. Either way, the speed measurements can be
regarded as suspect and not recorded for future use. If the speeds
are consistent, then there can be a high degree of confidence that
there was no interference, and the vehicle did travel at constant
speed and direction.
1TABLE 1 Peak Timing Algorithm for mp = 1 to 6 // mp = measurement
plane number { trigger_valid[mp] = 0; } nt = 0; // nt = number of
trigger values recorded repeat while speeds need to be calculated {
wait for the next sample period; i = i+1; for mp = 1 to 6 {
sample[mp] [i] = (the value from the analogue to digital converter
that monitors the measurement plane signal specified by the value
of mp as the appropriate light emitting elements are pulsed); j =
i-132; for k = j-128 to j+128 { med_9[mp] [k] = (the median value
of the 9 values from the sample array with indexes of [mp] [k-4] to
[mp] [k+4]); } for k = j-119 to j+119 { ave_19 [mp] [k] = (the
arithmetic average of the 19 values from the med_9 array with
indexes of [mp] [k-9] to [mp] [k+9]); } threshold[mp] [j] = (the
maximum value of the 239 values from the ave_19 array with indexes
of [mp] [j-119] to [mp] [j+119])/2; if threshold[mp] [j] < (the
minimum threshold value) { threshold[mp] [j] = (the minimum
threshold value); } if trigger_valid[mp] equals 0 { if ave_19[mp]
[j] > threshold[mp] [j] { trigger_time[mp] = j;
trigger_valid[mp] = 1; nt = nt+1; } } } if nt equals 6 { calculate
the speed from the trigger_time array values; for mp = 1 to 6 {
trigger_valid[mp] = 0; } nt = 0; } }
[0104] Modifications within the spirit and scope of the invention
may be readily effected by those skilled in the art. For example,
although the word light has been used above with the technical
meaning as commonly used in the fields of physics and engineering
(that is, visible light), the invention may also be implemented
with other forms of electromagnetic radiation such as infrared and
ultraviolet light. Therefore, in particular implementations of the
sensor system, the light-emitting elements may emit infrared or
ultraviolet light and the light-sensing elements may sense infrared
or ultraviolet light. It is to be understood, therefore, that this
invention is not limited to the particular embodiments described by
way of example hereinabove.
* * * * *